What vice can you just not seem to give up?

First off, I don't believe in guilty pleasures, because I don't think we should ever feel guilty about anything that makes us happy. That said, I also know that there are some things that aren't GOOD for us, whether for our bodies or our minds, or both.

One of those things is Diet Coke. It provides no nutritional value, and I far too often drink it instead of water, which I drink dreadfully little of throughout the day. (I'm constantly dehydrated, I'm convinced.) Yet, I can't give it up. It's my little mid-day pick-me-up.

But I can't be the only one. What vice can you not give up?